This invention relates in general to programmable logic controllers and, more particularly, to a method for converting an existing programmable logic controller hardware configuration and control program into another equivalent programmable logic controller hardware configuration and equivalent control program.
Programmable logic controllers (PLC's) are a relatively recent development in process control technology. As a part of process control, a programmable logic controller is used to monitor input signals from a variety of input modules (input sensors) which report events and conditions occurring in a controlled process. For example, a PLC can monitor such input conditions as motor speed, temperature, pressure, volumetric flow and the like. A control program is stored in a memory within the PLC to instruct the PLC what actions to take upon encountering particular input signals or conditions. In response to these input signals provided by input sensors, the PLC derives and generates output signals which are transmitted to various output devices to control the process. For example, the PLC issues output signals to speed up or slow down a motor, open or close a relay, raise or lower temperature or adjust pressure as well as many other possible control functions too numerous to list.
A typical PLC includes a rack into which a plurality of input/output cards may be placed. A rack includes several slots into which these input/output cards are installed. Each input/output card has a plurality of I/O points. An I/O bus couples the cards in the slots back to the processor of the PLC. The particular processor employed in a PLC together with the particular choice of input and output cards installed in the PLC rack are often referred to as the hardware configuration of the PLC. The hardware configuration also includes the particular addresses which the I/O cards employ.
As programmable logic controller technology progresses, it often becomes desirable to replace old programmable logic controllers (PLC's) with new programmable logic controllers. When it becomes necessary to configure a new programmable logic controller to be equivalent to an old or other programmable logic controller, both new equivalent hardware must be selected and new equivalent controller logic programming (control program) must be generated. Often, incompatibilities will exist in hardware rack sizes, I/O module densities, I/O point addressing and in programming constructs between the original and the new PLCs. There are often many complexities in the hardware selection process and in the logic programming of the control program.